Prepress
Prepress is where approved artwork becomes printable plates. The exam tests bleed and marks, imposition logic, separations, overprint and trapping, ink limits, PDF/X standards and preflight discipline.
Core rules
- Geometry: 3 mm bleed on all trimmed edges (more for heavy board or large formats), trim marks, and a safety margin of at least 5 mm from trim for text. Marks live in the slug, outside the bleed.
- Marks: crop marks, registration targets, a colour bar with solid and tint patches for densitometry and grey balance, and a job slug with name, date and version.
- Imposition: sheetwise, work-and-turn and work-and-tumble; 16-page signatures for book work; gripper and plate-cylinder allowances reduce the usable sheet.
- Separations: four process plates plus one plate per spot colour. Black text is normally set to overprint; white type knocks out. Registration black (100/100/100/100) is for marks only.
- Trapping: spreads and chokes of roughly 0.05–0.25 mm where two colours abut, to hide register variation on press.
- Total area coverage: about 300–330% for coated offset, 240–280% for uncoated, around 240% for newsprint.
- PDF/X-4 carries live transparency and ICC profiles; PDF/X-1a is flattened and limited to CMYK, greys and spots.
- Preflight: fonts embedded, image resolution at least 300 ppi at 100% (or 2 × lpi), correct colour spaces, hairlines no thinner than about 0.25 pt, TAC under the press limit, overprint preview checked.
- Proofing: contract proofs certified to ISO 12647-7, viewed under D50 illumination at about 2,000 lux per ISO 3664.
Worked example
A supplied A4 folder file (210 × 297 mm) arrives for coated offset. Page size must be 216 × 303 mm with 3 mm bleed. A shadow area uses C60 M40 Y40 K100 = 240% TAC, which is acceptable. A large solid background uses 100/100/100/100 = 400% TAC: reject it, because at that ink film the sheet will set off in the delivery pile and back-trap mottle on the next impression; rebuild it as 40/30/30/100 = 200%. A 7 pt black caption is set as a rich black build - reject and reset to 100% K overprint so register variation cannot fringe it.
Traps that cost marks
Leaving RGB images in a CMYK workflow; transparency unflattened for an old RIP; hairlines that vanish on plate; forgetting to overprint black text over photographs and getting white halos; rich black on small type; ignoring creep when imposing saddle-stitched sections.
Key points
- 3 mm bleed beyond trim; marks and slug outside the bleed.
- Registration black for marks only, never for type.
- TAC limits: ~300–330% coated, 240–280% uncoated.
- PDF/X-4 keeps transparency live; PDF/X-1a is flattened.
- Hairline minimum around 0.25 pt to survive plate and press.
- Proof under D50 at about 2,000 lux to ISO 3664.